Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Therein, a portion of an engine's cylinders may be disabled during selected conditions defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. An engine control system may disable a selected group of cylinders, such as a bank of cylinders, through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, and through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling. By reducing engine pumping losses, engine efficiency is improved.
One example approach for selecting cylinders for deactivation is shown by Springer et al in US 20130276755. Therein, cylinders are grouped based on compression ratio. As is known in the art, the “compression ratio” of an internal combustion engine is defined as the ratio of the cylinder volume when the piston is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC). In general, the higher the compression ratio, the higher the thermal efficiency and fuel economy of the internal combustion engine. In Springer, based on an engine load demand, one or more cylinders from a group having a specific compression ratio are selected for deactivation. In particular, a cylinder having a high compression ratio may be deactivated at a different load than a cylinder having a lower compression ratio. As an example, at part load operation, a group of cylinders having a lower compression ratio may be deactivated while the group of cylinders having a higher compression ratio may remain activated.
However, the inventors herein have recognized potential issues with such a system. As one example, due to the compression ratio grouping, the range of compression ratios available at part load operation may be limited. For example, in response to small load changes, the group of cylinders with the higher compression ratio may be deactivated and the group of cylinders with the lower compression ratio may be reactivated. Engine operation with the reduced compression ratio may result in a bigger fuel penalty than the fuel economy advantage of operating the engine with selective cylinder deactivation. In addition, even at part load operation, due to knock constraints, the group of cylinders with the lower compression ratio may have to be selectively activated. This is because compression ratios are limited by the availability of high-octane fuels needed to prevent combustion detonation or knock at medium-high engine loads.
The inventors have recognized that at least some of these issues may be addressed by engines having cylinders that are selectively deactivatable and that further include variable compression ratio mechanisms, such as pistons with variable displacement capabilities. Therein, by coordinating and synchronizing selective cylinder deactivation with adjustments to the compression ratio of the active cylinders, knock control and engine fuel economy may be improved. In one example, the synergistic benefits are achieved through a method for an engine comprising: selectively deactivating one or more engine cylinders based on engine load; and adjusting a piston displacement to vary a compression ratio of active engine cylinders to maintain spark timing of the engine at a peak torque timing while maintaining the one or more cylinders deactivated.
As an example, during conditions when engine load is lower, an engine may be operated in a variable displacement mode with one or more cylinders selectively deactivated (e.g., via individual cylinder valve deactivation mechanisms). Herein, each cylinder of the engine may include a piston coupled to a piston displacement changing mechanism that moves the pistons closer to or further from the cylinder head, thus changing the size of the combustion chambers. By changing the size of the piston displacement, the static compression ratio of the engine (that is, a volume of the cylinder when the piston is at Bottom Dead Center relative to the volume of the cylinder when the piston is at Top Dead Center) may be varied. After the partial cylinder deactivation, a piston displacement of the remaining active cylinders may be adjusted so that the active cylinders can be operated with a first, highest possible compression ratio while maintaining spark timing at peak torque timing (e.g., at MBT). As the engine load increases, and/or as the engine becomes knock limited, the piston displacement may be adjusted to reduce the compression ratio while maintaining the spark timing at the peak torque timing. Once the compression ratio reaches a second, lowest possible compression ratio, further indication of knock may be addressed by retarding spark timing from MBT until the fuel efficiency of operating with spark retard is equal to (or more than) the fuel benefit of the reduced pumping loss of having cylinders deactivated. Thereafter, further knock may be addressed by reactivating at least one of deactivated cylinders and resuming active cylinder operation at the first compression ratio and with spark timing advanced back towards MBT.
In this way, selective cylinder deactivation may be used to reduce engine pumping losses while concurrently varying a compression ratio via piston displacements to reduce engine thermal losses, providing synergistic benefits. The technical effect of synchronizing and coordinating the scheduling of the cylinder deactivation and changes to the piston displacement is that fuel economy can be significantly improved. In particular, by using piston displacement based compression ratio adjustments in real-time to address knock while one or more cylinders are deactivated, spark timing may be maintained at the peak torque timing for a longer duration of engine operation. By delaying the use of spark retard, and reducing the amount of spark retard needed to address knock, fuel economy and engine performance improvements are achieved.
The above discussion includes recognitions made by the inventors and not admitted to be generally known. Thus, it should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.